Retention Assessment of High Performance Poly-etheretherketone Removable Partial Denture Frameworks Constructed by Various Techniques (in vitro Study)

Statement of the Problem: Poly-etheretherketone is a novel material used in the construction of the removable partial dentures frameworks instead of the metal frameworks. This material can be fabricated by various techniques. Most common methods are the injection molding or Computer Aided Design/Computer Aiding Manufacturing (CAD/CAM) milling techniques. The fabrication technique may affect the adaptation of the frameworks by influencing the retention. Purpose: To assess the effect of the processing techniques of high performance Poly-etheretherketone either by injection molding (pressing) or CAD/CAM milling techniques on removable partial denture frameworks retention for rehabilitation of upper class I Kennedy classification. Materials and Method: This in vitro study was performed on one epoxy resin model representing the partially edentulous maxillary arch with natural teeth extending from first premolar to first premolar. First premolars and canines were reduced to receive porcelain fused to metal crowns with 0.50mm mesio-buccal retentive undercuts, distal guiding planes and mesial occlusal rest seat on first premolars and cingulum rest seat on canines. Considering the construction technique of frameworks, twenty samples were divided into two groups. In the group I, ten frameworks were fabricated by injection molding, and in the group II, ten frameworks were fabricated by CAD/CAM. The removal and insertion was carried out at 120, 720 and 1440 cycles for both groups, respectively. The retention values were measured by using Universal Testing Machine before cycling and after each interval. Results: Independent t-test showed significant difference on retention at different simulation cycles between groups. Group II exhibited significantly less retention than group I (p< 0.001), while comparing the retention at different cycles within each group by paired sample t-test exhibited significant decrease of retention till the end of the cycling (p< 0.001). Conclusion: From the retention point of view, high performance poly-etheretherketone frameworks fabricated by injection molding technique provided a promising method over CAD/CAM technique milling method.


Introduction
Nowadays, due to continuous development and advancement in polymer's industry fabrication, numerous materials have been introduced for dentures construction [1]. Currently, high performance thermoplastic pol-ymer polyetheretherketone (PEEK) has obtained increasing attention for dental applications; PEEK shows low density, good biocompatibility and provides dampening properties as well as certain ductility [2]. Moreover, it is characterized by high heat resistance, high me-chanical strength, and has a maximum degree of crystallinity (48%) and the low solubility (0.5%) and water absorption values under different aging solution [3].
High performance polyether-etherketone (PEEK) polymer (BioHPP) is used in construction of maxillary class I Kennedy classification removable partial dentures (RPDs) frameworks; it helps in diminishing the stresses and distal torque on abutment teeth [4]. Additionally, it achieves patient's desire of metal free RPDs and avoids the risk of oral galvanism [5]. BioHPP is a practicable biomaterial, which allows the replacement of conventional polymers, as well as the replacement of metals, ceramics, and alloys in the dentistry field.
BioHPP ® materials are available in both granulate and pellet form for injection molding technique also they are obtainable as milling blanks for the Computer Aided Design/Computer Aiding Manufacturing (CAD/CAM) technology. Nevertheless, in the field of dentistry, BioHPP may be processed by utilizing the well-known thermoforming technology [6].
In the last decade, great accomplishments were obtained particularly in prosthetic branch of dentistry concerning CAD/ CAM technology [7]. CAD/CAM technology with different techniques of production leads to the improvement of RPDs quality, furthermore, it increases the manufacturing efficiency outcomes [8].
Generally, BioHPP shows good milling properties and it is commonly the most used material in CAD/CAM technology [9]. This technology has no processes of wax pattern, investment, and casting, thereby avoiding oxidation firing cycle and allowing normal cooling rate.
It enables the holistic molding of RPDs frameworks via optimization of undercut position and size [10].
Injection molding (pressing) technique is one of the commonly utilized fabrication technologies in plastics industry [11]. In semi-crystalline polymer materials such as BioHPP, the temperature of mold is a corner stone in defining the parameters of the injected product for performance; BioHPP revealed high strength than numerous metals on a per mass basis. Previous study has suggested utilizing an exemplary temperature of the metal mold 175-220° C for semi-crystalline PEEK polymer [12]. Nevertheless, in the dental application, utilization of refractory products to form the mold is commonly used method to construct a positive reproduction for arches and associated structures [13]. Sufficient retention of RPDs is considered as one of the most important factors affecting their clinical success. Retention of RPDs is accomplished by placing clasp parts into undercuts on abutment teeth [14]. Some patients are not satisfied with their RPD, especially when it is not stable during mastication [15]. A previous study [16] investigated the initial retentive force of a BioHPP clasps to an abutment tooth; it was reported that retention force was less than that of chrome cobalt RDP frameworks. This is a matter of concern, especially with the use of a shallow under cuts equal to 0.25 mm ,which is the most routinely used depth of undercut, hence, it is essential to investigate the initial retentive force and retentive force using BioHPP clasps.
BioHPP materials have the prospect to engage and disengage undercuts without being stressed beyond their elastic limit. This opens up the possibility of prolonged and improved retention (over many years) for the RPDs frameworks fabricated from these novel materials. The in vitro repeated removal and insertion of frameworks, represents the simulation of many years of use. It also allows for a comparison of frameworks deformation following prolonged removal and insertion testing, which would be expected to have an undesirable effect on frameworks retention [17].
The aim of the current in vitro study was to assess and compare the effect of BioHPP processing techniques by either injection molding (pressing) or CAD/CAM milling technique on retention of RPDs frameworks used to rehabilitate maxillary class I Kennedy classification with a specific end goal to examine their retentive force. The null hypothesis was that the processing techniques have insignificant influence on the retention of BioHPP RPDs frameworks.

Material and Method
This in vitro study was performed on an epoxy resin study model to improve the standardization of the study, which represents maxillary class I Kennedy classification with teeth elongating from the first premolar on one side to the first premolar on opposite side. On this epoxy resin study model, twenty BioHPP maxillary Kennedy class I RPDs frameworks were constructed; ten frameworks by CAD/CAM milling technique, and ten frameworks by injection molding technique. Retention values were evaluated for each BioHPP framework.  In this study, there was no need to use conventional chrome cobalt RPDs as a control group as previous studies [16,18] established that BioHPP exerts fewer stresses on abutments compared to standard-alloy clasps, provides adequate retention, and satisfies aesthetic demands. These issues indicate that BioHPP presents a promising alternative to conventional metal clasps. Therefore, according to the technique utilized in the construction of BioHPP RPDs frameworks, the twenty frameworks were divided into two equal groups.
In the group I (n=10), BioHPP RPDs frameworks were fabricated by injection molding technique, and in the group II (n=10), BioHPP RPDs frameworks were fabricated by CAD/CAM technique.

Technique
Ten BioHPP RPDs frameworks were constructed by injection molding technique [4,19] and for 2 Press System (Figure 4a) in the following procedures.
The fitting of the ten 3D printed resin frameworks on the epoxy model were assessed and then the master model was duplicated into ten refractory cast to be used during the construction of the RPDs frameworks. Each printed resin frameworks was adapted to a refractory cast and the sprues were connected to it to act as a path of the melting BioHPP. Then, it was invested in a speci-  Following the complete melting of BioHPP granules, the press plunger was imported in the reservoir of the cylinder. The procedure was fully completed within 35 minutes; the mold was left to cool and divested or uncovered as usual. The BioHPP frameworks were disconnected from the sprues, and then they were finished and polished in regular manner.

Construction of BioHPP RPDs Frameworks by CAD/CAM Technique
The other ten BioHPP RPDs frameworks were constructed by CAD/CAM technique in the following steps [16,20]. Following the assessment of the 3D printed resin frameworks on the epoxy resin model, for each framework of the ten samples, the 3D design was introduced directly to manufacturing compartment to begin the milling process of BioHPP dental discs (BioHPP, Bredent GmbH) by using Ceramill motion 2 milling machine (Ceramill Motion 2, Amann Girrbach). The BioHPP framework was then removed from the disc by using carbide bur, then smoothed by removing any sharp angles and finally finished and polished in usual manner.

Evaluation of Retention Values of Both Groups Frameworks
The retention of the BioHPP RPDs frameworks was measured initially (0 cycles), then the retention was measured after being subjected to 120, 720 and 1440 insertion/ removal test cycles (ROBOTA chewing simulator integrated with thermo-cyclic protocol operated on servo-motor (Model ACH-09075DC-T, AD-technology CO., LTD., Germany) [21]. These cycles were corresponding to one month, six months and, one year, respectively by Universal Testing Machine (Shimazdu testing machine AG-X, 10N-10KN, Japan). This measurement was done by applying dislodging forces to the center of the frameworks in a vertical direction.
Three holes were made in anterior and posterior bar of major connector to allow attaching three pulling chains that were secured to metal ring hook to permit the pullout of BioHPP RPDs frameworks (Figure 4b).
This enabled the placement (insertion) of the framework to its predetermined terminal position and its subsequent  Table 1 reveals the descriptive statistics of mean retenti-  cycle. In addition, there was significant retention decrease at all insertion/ removal cycles 120, 720 and 1440 cycles, that corresponds to 1 month, 6 months and one

Results
year respectively.

Discussion
This in vitro study was designed to evaluate and com-   [27]. In addition, the highly elastic nature and flexibility of PEEK polymer might be beneficial in clasps designing, using deeper retentive undercuts on the remaining teeth, which consequently diminishes the denture pain resulted from excessive local pressure [12].
The higher undercut has been advocated by other studies [27][28][29] in order to overcome the lower removal force to dislocate the flexible clasps.
The abutment teeth (first premolars and canines) were prepared to receive PFM crowns because the porcelain material revealed the least wear amount, followed by enamel. A previous study [14], improved that the well-constructed glazed porcelain surface is able to withstand the forces of RPDs causing the wear of retentive clasp arms. These results are in agreement with those of Tietge et al. [30] and Marso et al. [31]. insertion/removal test cycles, there was a significant decrease in the retention values of both groups of the study, which may be due to the clasp deformity in both groups at these cycling intervals. In the same manner, the results of the current study propose that BioHPP RPDs frameworks clasps may lose their retention force value due to multiple deflections, which leads to continuous loss of elasticity [14].
It was found that each group exhibited significant decrease in retention between the initial (0 cycle), 120 cycle, 720 cycles and final retention (1440 cycles) due to significant wear occurred in all groups after simulating 3, 6 and 12 months of RPDs use. This might be a limitation of the present study since only the vertical forces exerted on the RPDs were evaluated and lateral forces affecting the retainer and the horizontal forces, which are to be expected during the chewing process, were not simulated [34].

Conclusion
Based on the study results, it could be concluded that:  From the retention point of view, PEEK RPD frameworks fabricated by injection molding technique considered a promising method over CAD/CAM technique milled method.  BioHPP RPDs frameworks clasps provide an optimal retention value when suitable retentive undercuts are provided.